Back seat

Will the back seat from a 72 Duster fit a 69 Dart?

No, the Duster has the 108" wheel base, while the Dart has an 111" wheel base. And also, the car lengths are different. The Duster rear seat is just a little narrower. Of course, a good upholstery shop could stretch, or modify the seat frame to fit, I believe. I have both, and there are a couple of inches difference.